The article mentions (in a "further research" kind of way) some of the obvious reasons that superhero associations specifically might elicit this response, when other pattern-disrupting surprises might not. But there's also this:

> If unexpected yet non-threatening events

Culturally, in some cities, some people would wonder whether the person in the Batman costume is mentally ill and a threat (how much, varying partly on how good of actors the two are, and how they play it). This will activate modes of thinking, possibly including thoughts of protecting others. Possibly compounding this, if the photo is during an experiment, then that suggests that, although "Batman" enters "from another door", he ends up standing right next to "pregnant woman", and therefore possibly a threat to the woman.

It would help to know how the person who gave up their seat then positioned themselves, and whether they kept an eye on Batman. Might also help to know how other people visibly reacted to Batman, other then generic unexpected event. Maybe you could get some sense of this with analysis of good video coverage, or, more subjectively, with one or more skilled intuitive observers.

Also, if the photo was obtained with a non-hidden camera, during the experiment, did people noticing someone taking photos affect their behavior?

Or could the prosocial behavior be prompted not by the unexpected merely disrupting one's internal patterns, but by one's awareness of others suddenly paying more attention (not everyone immersed in phones/tablets/books/earbuds) activating more prosocial behavior?

Or the disruption of the social context for others making it culturally easier to offer your seat? More distraction, shared experience in lighthearted moment, whatever.

(If you've been in the usual seat-giving-up situation in a crowded subway car, culturally, you might be almost embarrassed to offer your seat, not wanting the attention, nor to be seen as seeking praise, nor to embarrass anyone else who didn't give up their seat. You might start to get up, in such as way as to block a person who looks like they might grab it, and open up a path to the person you intend to offer it to, as you get their attention, maybe you say something like "ma'am" only loud enough for them to hear, as you gesture, they probably accept and say thank you, "no problem", then you walk away, as well as you can walk away in a subway car.)

I like that they didn't try to make it some subliminal superhero tie-in and instead proposed the more plausible mechanism of increasing attention to surroundings in the presence of something unusual.

Have read about 50-100 Psychology paper abstracts. This is the first time I've understood a Psychology paper abstract in the first read. Kudos for that.

Key learning - if you’re pregnant travel with a friend dressed as Batman